Organic photovoltaics (OPVs) is a rapidly growing area of research worldwide due to its promise to offer low temperature, inexpensive processing of lightweight and flexible solar cells. OPV cells based on organic polymers are of interest as alternative sources of renewable electrical energy to the typical silicon-based cell. Dramatic improvements in power conversion efficiency (PCE) of bulk heterojunction (BHJ) polymer-containing solar cells (PSCs) include recent reports on devices with PCE values exceeding 9%. (He, et al. 2012 Nat. Photonics 6, 591-595; You, et al. 2013 Nat. Commun. 4, 1410-1446; Liu, et al. 2013 Sci. Rep. 3, 3356; You, et al. 2013 Adv. Mater. 25, 3973-3978; Yao, et al. 2014 Adv. Energy Mater. doi:10.1002/aenm.201400206.)
However, achieving such high efficiency requires increasingly complex polymer syntheses and device architectures (e.g. fabrication of tandem devices). In addition, the use of aluminum as the most common metal cathode lacks practicality owing to its rapid oxidation and inability to be processed from solution. More stable metals, like Ag, Cu, or Au, can be deposited from solution, but have limited utility as cathodes in organic photovoltaics due to their high work-function (Φ) that further limits the open circuit voltage (VOC), short circuit current density (JSC), and fill factor (FF) due to low a built-in electrostatic potential difference across the device. (Krebs 2009 Sol. Energ. Mat. Sol. Cells 93, 465-475; Krebs, et al. 2009 J. Mater. Chem. 19, 5442-5451; Guo, et al. 2013 Adv. Energy Mater. 3, 1062-1067.)
To circumvent this limitation, a thin buffer layer inserted between the active layer and cathode tailors the interface, maximizes VOC, and minimizes contact resistance. Numerous inorganic buffer layers have been studied, such as Ca and LiF, while organic interlayers would be better suited to solution-based device fabrication. (Yip, et al. 2012 Energy Environ. Sci. 5, 5994-6011; Duan, et al. 2013 Chem. Soc. Rev. 42, 9071-9104; Gu, et al. 2014 Adv. Energy Mater. doi:10.1002/aenm.201301771.)
Conductive interlayers such as Ca, advantageous for their intrinsically low Φ, suffer from their relative lability and sensitivity to oxygen or water. Polar organic interlayers permit layer-by-layer solution deposition, but have poor adhesion to low surface energy active layers, thus limiting their utility in conventional device architectures (as fabricated from anode-to-cathode). (Zhang, et al. 2013 J. Mater. Chem. A 1, 9624-9629.) Furthermore, buffer layers are typically very thin (<5 nm), so as to prevent charge-build up due to large injection barriers at the active layer/buffer layer interface or slow charge transport through the buffer layer. However, from a processing standpoint, the need to reproduce precise nanometer or sub-nanometer interlayer thicknesses is in itself problematic.
Buffer layers, or interlayers, lower the work function of the cathode, with a magnitude frequently described by the interfacial dipole (Δ), where large negative Δ values have produced some of the most effective reported OPVs. (Worfolk, et al. 2012 Adv. Energy Mater. 2, 361-368.) For example, solution-processed dimethylaminopropyl-substituted polyfluorene (PFN) yielded a maximum PCE of 9.21% in an inverted device, while poly(ethyleneimine) (PEI) and its derivatives enabled all-solution-processed inverted devices with maximum PCE values of 8.9%. (Zhou, et al. 2012 Science 336, 327-332; Woo, et al. 2014 Adv. Energy Mater. doi:10.1002/aenm.201301692.) In each case, the amine functionality of the interlayer is responsible for the large negative Δ values (<−0.5 eV). However, these interlayers have their own drawbacks—the PFN backbone is intrinsically p-type, while PEI is insulating and exhibits poor adhesion to the photoactive layer.
With respect to electrode selection, recent reports of BHJ PSCs using a bathocuproine (BCP) interlayer with a Ag cathode achieved PCEs of 7.7 and 8.1%, representing benchmark values to-date for standard single-junction PSCs containing Ag cathodes. (Martinez-Otero, et al. 2013 Adv. Optical Mater. 1, 37-42; Betancur, et al. 2013 Nat. Photonics 7, 995-1000.) However, BCP requires a thermal deposition step and a precisely defined interlayer thickness (3.5 nm) to be effective. (Martinez-Otero, et al. 2013 Adv. Optical Mater. 1, 37-42.) Conjugated polymer zwitterions (CPZs) were recently reported that show large negative Δ values (−0.5 eV to −0.9 eV) on metal electrodes. (Page, et al. 2013 Macromolecules 46, 344-351; Liu, et al. 2013 Adv. Mater. 25, 6868-6873; Page, et al. 2014 Chem. Sci. doi:10.1039/c4sc00475b.) Spin-coating CPZs and the active layer polymer from orthogonal solvents provides good control over interlayer thickness with little disruption of the underlying surface. To date, CPZs have demonstrated effectiveness as interlayers in OPV devices over a thickness range of ˜5-10 nm; however, thicker films are not useful due to the p-type characteristics of the selected polymers. (Liu, et al. 2013 Adv. Mater. 25, 6868-6873.)
Inverted polymer solar cells (iPSCs) containing high work function metal anodes (e.g., Ag or Au) and modified indium tin oxide (ITO) cathodes exhibit superior efficiency and stability over PSCs with a conventional geometry. (He, et al. 2012 Nat. Photon. 6, 591; Chen, et al. 2009 Adv. Mater. 21, 1434; Hau, et al. 2010 Polym. Rev. 50, 474; Jorgensen, et al. 2012 Adv. Mater. 24, 580; Liu, et al. 2013 J. Am. Chem. Soc. 135, 15326; Zhang, et al. 2014 Adv. Energy Mater. DOI: 10.1002/aenm.201400359.) iPSCs preclude the need for a poly(3,4-ethylenedioxythiophene):poly(styrenesulfonic acid) (PEDOT:PSS) hole transport layer, which is corrosive to ITO and leads to device deterioration. (Xu, et al. 2009 Adv. Funct. Mater. 19, 1227; Yang, et al. 2012 Adv. Energy Mater. 2, 523; Jorgensen, et al. 2008 Sol. Energ. Mat. Sol. C. 92, 686.)
A major limitation associated with iPSCs is the large barrier to electron extraction at the photoactive layer-ITO interface. To address this limitation, inorganic materials are implemented as electron transport layers (ETLs), including zinc oxide (ZnO), cesium carbonate (Cs2CO3), titanium oxide (TiOx), and titanium chelate. (You, et al. 2012 Adv. Mater. 24, 5267; White, et al. 2006 Appl. Phys. Lett. 89, 143517; Sun, et al. 2011 Adv. Mater. 23, 1679; Li, et al. 2006 Appl. Phys. Lett. 88, 253503; Liao, et al. 2008 Appl. Phys. Lett. 92, 173303; Waldauf, et al. 2006 Appl. Phys. Lett. 89, 233517; Tan, et al. 2012 Adv. Mater. 24, 1476.) However, organic ETLs possess inherent advantages over inorganic layers for their ease of processing and favorable mechanical properties. (Yip, et al. 2012 Energy Environ. Sci. 5, 5994; Duan, et al. 2013 Chem. Soc. Rev. 42, 9071.) Prime examples of organic ETLs used for ITO modification in iPSCs include polyfluorene derivatives (PFNs), polyethyleneimine (PEI), and ethoxylated polyethyleneimine (PEIE). (He, et al. 2012 Nat. Photon. 6, 591; Liu, et al. 2013 J. Am. Chem. Soc. 135, 15326; He, et al. 2011 Adv. Mater. 23, 4636; Kang, et al. 2012 Adv. Mater. 24, 3005; Lee, et al. 2013 Energy Environ. Sci. 6, 1152; Zhou, et al. 2012 Science 336, 327.) In these devices, the amine groups impart a large negative interfacial dipole (Δ) (<−0.5 eV) that reduces the energy barrier to charge extraction and increases the built-in potential of the device. However, these ETLs have drawbacks—the PFN backbone is intrinsically p-type, whereas PEI and PEIE are insulating and exhibit poor adhesion to the active layer.
Interest is thus emerging in fullerene-based ETLs that promote rapid electron transport and selectivity, and provide the capacity for π-π interactions to enhance adhesion with the active layer. (Yao, et al. 2014 Adv. Energy Mater. 4, 1400206; O'Malley, et al. 2012 Adv. Energy Mater. 2, 82; Yang, et al. 2013 Adv. Energy Mater. 3, 666; Wei, et al. 2008 Adv. Mater. 20, 2211; Mei, et al. 2013 ACS Appl. Mater. Interfaces 5, 8076; Li, et al. 2013 J. Mater. Chem. A 1, 12413; Li, et al. 2013 Adv. Energy Mater. 3, 1569; Lai, et al. 2013 ACS Appl. Mater. Interfaces 5, 5122.) However, only a few examples of fullerene based ETLs are sufficiently robust to endure multilayer solution processing, including thermally cross-linked fullerene derivatives, a phosphoric diethyl ester functionalized fullerene, a fullerene/ZnO composite, and a blend of fulleropyrrolidinium iodide (FPI) and PEIE (FPI-PEIE). (Hsieh, et al. 2010 J. Am. Chem. Soc. 132, 4887; Duan, et al. 2012 Chem. Mater. 24, 1682; Cheng, et al. 2013 ACS Appl. Mater. Interfaces 5, 6665; Liao, et al. 2013 Adv. Mater. 25, 4766; Liao, et al. 2014 Sci. Rep. 4, 6813; Li, et al. 2014 Adv. Mater. 26, 6262.)
Realizing uniform ultrathin films over large areas represents a significant challenge, yet most efficient iPSCs reported to-date require an ultrathin ETL (e.g., ˜5 nm of PEIE or PFN). While a recent report describing the incorporation of mercury into PFN (PFEN-Hg) achieves thickness independent properties, mercury carries inherent practical limitations. (Liu, et al. 2013 J. Am. Chem. Soc. 135, 15326.) Doping FPI with PEIE (FPI-PEIE) also leads to an ETL thickness independence, but the doping ratio needs to be elaborately controlled. (Li, et al. 2014 Adv. Mater. 26, 6262.) Simpler materials are thus needed to improve the properties of large area coatings while maintaining device efficiency.
Accordingly, there remains an urgent, on-going need for novel materials, methods and designs to enable improved power conversion efficiencies of OPVs, especially novel approaches to interlayers for polymer-based solar cells.